Cost-effective and minimally invasive monitoring of metastatic breast cancer (MBC) patients for disease progression or response to therapy is an unmet need in the clinical management of the disease. Imaging technologies are currently the gold standard of such monitoring. However, there is controversy regarding the type & frequency of imaging required. Imaging is expensive, time consuming, slow to detect disease changes and raises concern about repeated radiation exposure and costs are not always covered by insurance. Complementary approaches to imaging such as circulating tumor cells and biomarkers are used in the Standard of Care (SOC), although their application remains limited. A simple, cost-effective blood test to measure the level of biomarkers which are drivers of the disease aggressiveness should provide novel solutions for real-time monitoring of therapy response and reduce dependency on imaging in the MBC population. The PI has identified an 88kDa glycoprotein GP88 (progranulin), elucidated its biological activity as an autocrine growth & survival factor. GP88 is secreted from cancer cells and measurable in biological fluids at higher levels in breast cancer patients, compared to healthy individuals. Based on these characteristics, an innovative diagnostic test (Enzyme Immunoassay ? EIA) to measure GP88 in circulation was established. Our SBIR Phase 1 retrospective study demonstrated a statistical association between serum GP88 levels measured by the EIA test and objective measures of disease, i.e. RECIST 1.1 in 101 MBC patients. We established that a serum GP88 level of 56ng/mL was a stratification point below which patients have improved overall survival while patients with GP88>56 ng/ml have a poor outcome. We showed that serum GP88 levels were statistically correlated to response to therapy in addition to progression of disease unlike the SOC biomarker CA15-3 which is only associated to progression of disease. Based on these promising data, we are proposing an SBIR Phase 2 prospective longitudinal study over a 15-month period enrolling 120 MBC patients at two clinical sites by measuring serum GP88 level every month with imaging performed every 3 months. The objectives are to validate the findings of phase 1 and establish the predictive use of the GP88 test in monitoring MBC patients for disease progression/ response as an adjunct to imaging and aid in the clinical management of such patients. Specifically, we will (1) Validate that serum GP88 levels are correlated to disease response/ progression as defined by imaging results; (2) Examine the impact of successive high (>56ng/mL) and low (<56ng/mL) GP88 blood levels on future response/ progression and time to progression as defined by imaging results; (3) Validate that patients with consistent low (<56ng/mL) GP88 blood levels, have improved survival compared with patients with consistently high (>56ng/mL) GP88 blood levels. The outcomes of this phase 2 will be to establish clinical data to support commercialization of GP88 EIA as an integral part of clinical management of MBC patients that will contribute to improved care of MBC patients at reduced cost.